Dr. Paula Szkody[1] of the University of Washington and collaborators are performing a comprehensive study of pulsating white dwarfs in cataclysmic variable systems using both ground and space-based observations. In the last week of October, they will use the Hubble Space Telescope's functional component (the Solar Blind Channel) of the Advanced Camera for Surveys to obtain time-resolved ultraviolet spectroscopy of the cataclysmic variable SDSS 074545. These observations serve two purposes: to look for time-variability in the ultraviolet spectrum assocation with the pulsations of the white dwarf, and to use the overall spectrum to fit a temperature to the white dwarf itself. Detection of time-variability would help to define the pulsation periods of the white dwarf, which in turn provide information about the white dwarf's structure; an accurate measurement of the temperature would help white dwarf seismologists to define the white dwarf instability strip -- the region of the Hertzsprung-Russell diagram[2] where white dwarfs are capable of maintaining self-excited pulsations.

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Figure 1. A theoretical H-R diagram showing the location of several types of pulsating variable. The white dwarfs follow the cooling sequence on the left-hand side of the diagram. (Image from J. Christensen-Dalsgaard, Lecture Notes on Stellar Oscillations, Fifth Edition, May 2003; available from www.phys.au.dk/~jcd/oscilnotes/[4])

In order to observe this target safely, the controllers at the Space Telescope Science Institute must know that SDSS 074545 is at its quiescent level, or is fainter than a certain limit. AAVSO observers are able to make these observations, and your help is urgently needed to facilitate these important HST observations.

Observers are requested to perform nightly observations of SDSS 074545 in the two weeks prior to the HST observing window, and intensive observations during the window. Since SDSS 074545 is at V=19.1, this observation will be as challenging as those of the previous HST target, PQ Andromedae. The image below is a 4.5x4.5 arcminute V-band image with a 5-minute exposure time taken with the USNO 1.0-meter. This object has never been observed in outburst before, so its maximum brightness is unknown. If you cannot detect SDSS 074545 in reasonable time, please follow the observing guidelines below.

Figure 2. A 4.5'x4.5' V-band image of SDSS 074545 obtained with the USNO-1.0m telescope in a 5-minute exposure. Depending upon your observing system, target either SDSS 074545 itself, or either of the two comparison stars given below. Detection of the comparisons but not SDSS 074545 will be sufficient proof for HST to initiate observations of this target. If necessary, take multiple short exposures and stack them to reach the required S/N in SDSS 074545, or in either of the two comparison stars used for fainter-than limits.

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Requested Observations

Observers are asked to monitor SDSS 074545 (full name: SDSS J074531.92+453829.5: RA: 07h 45m 31.92s, Dec: +45d 38m 29.5s, J2000) beginning immediately (2007 October 19 UT). Visual observers please observe this object as normal, and report the faintest comparison star magnitude you can detect if unable to reach SDSS 074545 itself. CCD observers are asked to use filters during observations if available; V filter is preferred, but B,Rc, and Ic may also be used. Beginning 2007 October 29, please observe this object as often as possible through November 03 UT using the observing procedure outlined below, and submit data as quickly as is possible. In the event of an outburst, please contact the AAVSO immediately.

For CCD observers, if you are capable of reaching V=19.1 in reasonable time by stacking or direct integration, please obtain a S/N of 10 in SDSS 074545 itself. Alternately, please expose so that you can make a "fainter-than" determination with a S/N of 10 in at least one of the comparison stars. All of the comparison stars fainter than 14.5 are acceptable, but we recommend using the V=15.8 comparison star (AUID 000-BCY-392; RA=07h 45m 35.45s, Dec=+45d 38m 47.6s) for a fainter-than estimation. If you can go fainter and still reach S/N of 10 in reasonable time, please do so.

Again, we emphasize that both positive and fainter than observations are useful as long as the fainter-thans can reach the limits outlined above. If you are capable of detecting SDSS 074545 itself, please try to do so as we can then detect the start of any outburst; but fainter-thans are also very important constraints for the HST observation planners.

Please note that as of October 19, 2007, charts cannot be generated using the assigned name "SDSS074545", but they can be using either the designation (0738+45) or the coordinates: RA 07:45:31.92 , Dec +45:38:29.5 (J2000). We will correct this issue as soon as possible. Further announcements on this campaign will be issued via AAVSO Special Notices over the next 7-10 days.

This campaign is being coordinated by Matthew Templeton at AAVSO headquarters.